A water quality analyzer for measuring residual chlorine concentration in tap water is disclosed in Japanese Laid-open Patent Publication No. 2002-214220. The water quality analyzer has a pair of electrodes made of different metals and inspects chlorine concentration in a liquid of inspecting object from an electromotive voltage generated across the electrodes with the electrodes soaked in the liquid. One of the electrodes is made of platinum wire, while the other is made of silver wire and has silver chloride coat at its part soaked in the liquid. The pair of electrodes constitute a sensor. When the sensor is soaked in a liquid of inspecting object, for example, tap water, an electromotive voltage is generated across the pair of electrodes in response to chlorine concentration in the liquid.
Thus, chlorine concentration in a liquid of inspecting object is measured based on an electromotive voltage generated across the pair of electrodes when the electrodes is soaked in the liquid. However, detection precision of the chlorine concentration was low due to large dispersion in the electromotive voltages among those individual sensors. FIG. 18a shows electromotive voltages of sensors that are soaked in a liquid (clean water) with chlorine concentration of 0% and liquids (tap water) with 0.4 ppm (residual chlorine concentration), 0.9 ppm and 1.5 ppm. There was a tendency that the electromotive voltages saturated in high concentration side, and also dispersion in the electromotive voltages was large among the sensors. For example, the chlorine concentration generating the electromotive voltage of 325 mV disperses between 0.6 ppm and 1.5 ppm (see d1 in FIG. 18a).
It is considered that such dispersion in the electromotive voltages is brought by factors such as crack occurred in silver chloride coat on the one electrode surface or the like. FIG. 17 is an equivalent circuit diagram of a sensor 1 comprised of the pair of electrodes. The inventors considered that a voltage source E generating an electromotive voltage was connected in parallel with an impedance element Z2 by crack occurred in the silver chloride coat or the like, so that the dispersion in the electromotive voltages was brought by dispersion among individual sensors in an impedance value of the impedance element Z2. Incidentally, Z1 in FIG. 17 is an internal impedance of the sensor.
In this way, while electromotive voltages generated across the electrodes have dispersion among individual sensors, the dispersion in the electromotive voltages is remarkable in high concentration side and an inclination of the electromotive voltages is substantially the same among the sensors. Accordingly, when a constant voltage is added to or subtracted from electromotive voltages of each sensor in a range of high concentration side so that the electromotive voltage of the sensor soaked in a liquid of, for example, 0.4 ppm becomes a prescribed reference voltage (e.g., 200 mV), it is possible to calibrate electromotive voltages of each sensor. However, in this calibration technique (hereinafter referred to as a shift calibration), effect on reduction of the dispersion in the electromotive voltages was insufficient. FIG. 18b shows electromotive voltages after the shift calibration. Since chlorine concentration generating, for example, the electromotive voltage of 325 mV has dispersion between 0.8 ppm and 1.55 ppm (d2 in FIG. 18b), further improvement of inspection precision is required.
It was also not possible to detect water quality of a liquid of inspecting object at high precision since linearity of electromotive voltages generated across the electrodes was deteriorated by the tendency that the electromotive voltages saturated in high concentration side, as mentioned above.